The present invention relates to vehicle engine charging systems, and more particularly, to such systems of the type including both a mechanically-driven supercharger (such as a Roots type blower), and an exhaust gas-driven turbocharger. Vehicle engine charging systems of this type are sometimes referred to, for simplicity, by the term “superturbo”.
As is well known to those skilled in the art of vehicle engine boosting, there are two primary types of boosting devices typically used to boost the air pressure in the combustion chamber of internal combustion engines. The first is typically referred to as a “supercharger” (although that term is sometimes used generically for all boosting devices), and for purposes of the present specification, the term “supercharger” will be understood to mean and include a mechanically-driven charging (air pumping) device, i.e., the supercharger is driven at a speed which is normally proportional to engine speed. The second type of boosting device is an exhaust gas-driven turbocharger, i.e., a device including an exhaust gas-driven turbine which, in turn, drives a pump (compressor). Therefore, a turbocharger is driven at a speed generally proportional to the flow of exhaust gas from the engine exhaust manifold.
The typical mechanically-driven supercharger comprises a positive-displacement pumping device, such as the Roots-type blower, sold commercially by the assignee of the present invention. A Roots-blower supercharger is typically driven, off of the engine crankshaft, by a pulley and belt arrangement. In recent years, it has become more common for those skilled in the supercharger art to interpose a clutch between the engine crankshaft and the input shaft of the Roots blower to reduce noise, vibration and harshness (NVH) and avoid overspeeding the supercharger.
As is also well known to those skilled in the engine supercharger art, it is typical for the Roots-type blower to be provided with a “bypass” passage from the Roots blower outlet, permitting air flow back to the inlet. Within the bypass passage, there is normally disposed some sort of bypass valve, such as a butterfly-type plate valve, operable to move between a closed condition (blocking bypass flow back to the inlet), and an open condition (allowing blower outlet air to flow freely back to the inlet. As is further well known, having the bypass valve in the open condition effectively “unloads” the Roots blower, allowing a certain amount of the air from the outlet to re-circulate back to the inlet, thus reducing the boost pressure of air flowing to the intake manifold, as well as the amount of engine horsepower consumed by the blower, although still requiring more input horsepower than if the clutch were disengaged.
In the typical “superturbo” boosting system, including both the supercharger and the turbocharger, the system would normally be arranged with the supercharger disposed “upstream”, and the turbocharger disposed “downstream”, i.e., in terms of air flow direction from the intake, past the throttle, and into the combustion chamber. In such a superturbo system, boost is provided primarily by the supercharger at relatively low engine speeds, when there is not yet sufficient flow of exhaust gas to drive the turbocharger. Then, as engine speed increases (medium engine speeds), and as there is sufficient flow of exhaust gas to drive the turbocharger, both the supercharger and the turbocharger, together, provide boost to the engine.
As the assignee of the present invention has been engaged in development of a superturbo system, several observations have been made. First, the general functionality of the known arrangement of superturbo systems is generally acceptable, in terms of achieving boost using the supercharger when that makes sense, and achieving boost using the turbocharger when that makes sense, i.e., matching engine need to capability of each type of boosting device. Second, it has been observed that supercharger engagement can be objectionably harsh due to the sudden and large change in speed of the supercharger (high angular acceleration) as the supercharger clutch is engaged, especially at the lower end of the load range and the higher end of the speed range where the supercharger is operational. Furthermore, when the supercharger clutch is disengaged (e.g., at engine speeds above about 3,500 rpm or below 3,500 rpm when the demanded load is low enough to be covered solely by the turbocharger), and then engine speed drops, and thereafter it is desired to again operate the supercharger, the re-engagement of the supercharger clutch (for example, with the supercharger input stationary, and the clutch turning 2,500 rpm) would typically occur in a sudden manner likely to result, over time, in reduced durability of the clutch, and observed to cause an undesirable NVH condition.